Characterization of the photodynamic behavior of extrinsic fluorescent probes of biological systems is the purpose of the work described in this proposal. Specifically, the effects of various solvent parameters (viscosity, polarity, etc.) on the rates of internal conversion and intersystem crossing from the electronically excited states of the probes will be measured. Such information, backed by a parallel study of time resolved emission spectra, fluorescence quantum yields, and fluorescence lifetimes will provide a basis for a detailed understanding of the information about the interaction of the solvent or membrane environment with the fluorescent probe. This project obtains novelty by the new technique introduced for measurement of intersystem crossing quantum yields and by the completeness of the photodynamic characterization of the dyes. This will enable much more effective use of probes. Satisfactory resolution of each molecular characteristic will foster greater insight into the workings of the probe environment and thus increase our understanding of the structural and dynamic features of biological membranes.